Many prior art power on reset circuits exist of which a majority are meant for digital applications for which the reset needs to release once the power supply has reached a level safe for the digital circuitry to store voltages for a known startup condition. Thus the reset level can effectively track the threshold voltages (VT's) of the MOS devices and can vary with the large VT process variations (3:1 in some cases over temp and process). If analog circuitry exists there may be a need for a more accurate reset so that reliable operation of the analog circuits is achieved. This is often a more complicated function than VT variation and may require an accurate band-gap based power on reset, which is stable over process and temperature variations.
FIGS. 1 and 2 show examples of prior art power on reset circuits.
The circuit of FIG. 1 can be designed for low power applications with a current consumption of about 10 nA with a total resistance of about 80 Mohms. The circuit functions well for digital applications and has a well-known VT dependence for the power supply threshold. The trip point can vary from 0.8V down to 0.2V typically. If the circuit were used for analog applications as simple as an internal clock oscillator, the oscillator could malfunction corrupting the on chip clock and subsequently corrupting the digital data that the clock system drives. This is especially problematic if the circuit undergoes a sudden power loss.
The circuit of FIG. 2 is an example of an accurate band-gap comparator. This solution combines the function of a band-gap reference with the supply tap comparator. Resistors R2 and R3 are the supply sampling string resistors and the supply voltage VDD is divided down by R3/(R3+R2). This divided down supply goes into the band-gap comparator which switches state when the input (bases of transistors Q0 and Q1) are equal to the band gap voltage. Transistor Q1 is run at a lower current density than transistor Q0, and resistor R0 completes the well known proportional-to-absolute-temperature (PTAT) loop. Resistor R1 thus has a PTAT voltage across it (at the trip point) and transistor Q0's base-to-emitter voltage (vbe) completes the band gap voltage at the bases of transistors Q0 and Q1. The resistors must be sized properly for a given technology to make the comparator stable over process and temperature.
One drawback to this design is the required voltage headroom for operation is greater than the band-gap voltage (˜1.2V). The other drawback is the need for several large resistors since low current consumption is desired. The total resistance can total 100's of Mohms for current consumption of 10's of nA. This creates a significant die size penalty.